Alum (Fitkiri) is a group of chemical compounds, typically double sulfates of aluminum and another metal, such as potassium or ammonium. The most common form is potassium-alum (KAl(SO₄)₂·12H₂O), but there are other varieties like ammonium-alum (NH₄Al(SO₄)₂·12H₂O). Alums have many uses across different industries, including: Water purification: They help to clarify water by causing particles to coagulate and settle. Cosmetic products: Alum is found in some deodorants and aftershaves due to its astringent properties. Food preservation: It is used as a pickling agent and to maintain the crispiness of fruits and vegetables. Antimicrobial properties: Alums are sometimes used for their mild antibacterial effects.